System for controlling oscillator grid current



Jan- 1 1954 c. E. ELLSWORTH 2,

SYSTEM FOR CONTROLLING OSCILLATOR GRID. CURRENT Filed March 5, 1948 //v VE/v TOR C494 ffZLS/VORT/r AT TORNEYS Patented Jan. 12, 1954 SYSTEM FOR CONTROLLING OSCILLATOR GRID CURRENT Carl E. Ellsworth, Louisville, Ky., assignor, by mesne assignments, to National Cylinder Gas Company, Chicago, 111., a corporation of Delaware Application March 5, 1948, Serial No. 13,275

13 Claims. 1

This invention relates to high-frequency heating systems employing oscillators for generating the high-frequency energy and has for an object the provision of an improved grid-control system which assures increased efficiency of operation of the oscillator tube or tubes by maintaining the grid excitation more or less independent of the characteristics of the load presented to the oscillator.

In the more common types of oscillator circuits, grid excitation is often a function of frequency and more often varies with the power output in a manner which decreases the emciency of the oscillator. In high-frequency systems used for raising the temperature of dielectric materials, such as plastic preforms and the like, the load characteristics vary substantially from the beginning to the end of the heating cycle, thus affecting the output and often the frequency of the oscillator. In the heating of a typical plastic preform load, for example, the

oscillator plate current is usually initially adjusted somewhat below the full load rating so that the tube will not be overloaded toward the end of the heating cycle due to increase of the loss factor (power factor times dielectric constant) with increasing temperature: if the grid current be adjusted to optimum value at the beginning of the heating cycle, it will decrease to undesirably low value at the end of the heating cycle. Accordingly, the oscillator operates most inemciently at the time of maximum power output, causing undue dissipation of energy at the anode of the oscillator tube. When, on the other hand, the dielectric material is one whose loss factor decreases with increasing temperature;

the grid current must be adjusted below its optimum value at the beginning of the heating cycle, so that it will not rise beyond a safe value toward the end of the cycle.

in a sense and extent to maintain the grid exci- V tation at its predetermined optimum value.

In one form of the invention, the reactance may be a coil which is mechanically elongated or shortened by a solenoid, or equivalent device, in response to changes in magnitude of the grid exci-" tation so to change the grid-circuit inductance and thus control the grid excitation. In other forms of the invention, the grid coil may be of fixed mechanical dimensions and its inductive reactance varied by movement of a core or vane structure actuated by a solenoid or similar electromagnetic device responsive to changes in the grid current.

For a more detailed understanding of the invention and for illustration of various forms thereof, reference may be had to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 diagrammatically shows a dielectric heating system including one electro-mechanical arrangement for varying the grid-circuit reactance; and

Figs. 2 and 3 are isometric views of other electro-mechanical arrangements of varying the reactance of the grid circuit.

Referring to Fig. 1 of the drawings, the invention in one form has been shown as applied to a high-frequency heating system utilized for elevating the temperature of plastic preforms l8 supported on a tray I I conductively supported by a stationary plate l2. Above the preform ill or other dielectric load is a second plate or electrode 13 which may be adjustable or preset to provide an air space between it and the plastic preform I0. The electrode is may be adjustably connected to an output coil I l of an oscillator. The oscillator tube [5 may be connected to the usual filament transformer l6 and to a source of anode current Whose positive and negative terminals have been identified respectively as 13-!- and B. A radio-frequency choke-coil I! is included in the direct-current anode circuit and the high-frequency output of the tube 15 is fed by way of the coupling capacitor ill to the load circuit. The inductance of the coil i4. together with the capacitance between the electrodes II and I3, forms a tuned anode or resonant frequency-determining circuit for the oscillator. The grid circuit of the tube I5 receives its excitation through the anode-grid capacitance indicated by the dotted line connection including the coupling capacitor it. Where this interelectrode capacitance is not adequate, a capacitor of the desired size may be connected in shunt thereto.

The grid circuit includes an adjustable reactance shown in the form of inductance coil 26 anchored at one end to a supporting insulator 2|. The free end of coil is mechanically connected by an insulator 22 to the movable core 23 of a solenoid 24. So far as the high-frequency currents are concerned, the grid circuit comprises the inductance 20, flexible lead 25 and by-pass condenser 26, all in series between the grid and cathode of tube It. The direct-current component of the current in the grid circuit flows through the inductance 2c, and thence through flexible lead 21, resistor 28 and the winding of the solenoid 24 to the grounded cathode of tube I5. Conventional lay-pass capacitors 30 and 3| are included in the filament circuit to provide a low impedance path for the high.- frequency currents to ground.

In operation, to elevate the temperature of. the plastic preform ill, or other suitable dielectric load, it will be assumed that the, electrodes 1.! and I3 have their proper spacing and that the. adjustable connections to the output coil M are in proper positions for optimum operation of the oscillator at a predetermined frequency. which may be of the order of from 1 to 50 megacycles, to 25 megacycles being the usual operation of high-frequency systems of this character. It will be furtherassumed that the D. C. component of the grid current has the proper value for producing the optimum bias on the grldof the tube [5. This may be accomplished by proper selection of the value of the resistor 28 and of the initial setting of the inductance 20. With the oscillator. adjusted for optimum efficiency of; operation, the temperature of the plastic preforms ID will be rapidly increased. However, during the heating of such dielectric materials, the dielectric constant and the power factor may rise, thereby decreasing the, frequency and increasing the power absorbed by such dielectric material. Both the decreased frequency and the increased power output cause. the grid drive or grid excitation to be decreased, thereby decreasing the D. C. comppnent of the grid current. The increased power absorption by the load causes a decrease in the radio-frequency voltage of the tank, circuit including the output coil !4 and the capacitance between electrodes H and i3. Thus, the decreased high-frequency voltage of the tank. circult results in a decrease in the high-frequency voltage or feed back applied through the capacitor t9 to the grid circuit. Because of decreased,

excitationof the grid circuit, less high-frequency YQltage will be rectified and thusthe direct-current component will decrease. The cumulative result of the foregoing is that the direct-current biasing voltage of the grid decreases and the plate dissipation of the tube rises, with consequentinefficient operation of the oscillator. The foregoing effects may continue until, at the end of the, heating cycle, the grid bias and the plate dissipation are outside the limits specified by the tube manufacturer and outside the desirable and safe operating ranges for the tube. Under such conditions, the oscillator operates most inefficiently at the end of the cycle and at the very timeit isoalled upon to deliver its maximum load. Under conditions of maximum demand, it is 'most essential, from the efliciency and tube life Standpoints to operate the tube within safe limits, and preferably within the ranges specified by the tube manufacturer.

"Such excessive anode dissipation at the endof the heating cycle may be avoided by proper selection of the value of resistor 28 and of the setting of inductance 20, but in such casethe adjustments will ordinarily result in excess grid dissipation at the beginning of the cycle. In actual operation of systems not utilizing the present, invention, these extremes are avoided by manual adjustments made from time to time during the course of ant or operator. This supervision is not required the heating cycle by an a ttend--.

in arrangements utilizing the invention and the required adjustments to avoid either excessive grid dissipation or excessive anode dissipation are made automatically as the need arises.

In accordance with the present invention, the direct-current component of the grid current may be utilized to energize the solenoid 24 whose plunger or armature 23 may be made effective to adjust the inductance of coil 20 by an amount adequate to maintain the grid excitation within the range specified by the tube manufacturer and within a relatively narrow optimum range of grid excitation for most efficient operation of the tube IS.

The coil 20 is preferably constructed of a good elastic electrical conductor, so that the turns of the coil 20 themselves form a spring against which the plunger 23 acts. The coil 20 may be of beryllium-copper or any other suitable material, and it may be edge-wound from strip material, or. it may be wound of a round conductor. In any case, the adjustment of the solenoid 24 and the coil 20 will be such that the optimum value of inductance is established by thev positioning of the turns thereof for the optimum grid bias of the tube l5 as effected by a predetermined magnitude of the D. C. component of the grid current. It will be further understood that as the electrical parameters of the dielectric load change, there will be maintained a balance. between the pull of the solenoid 24 and the spring bias exerted by the turns of the coil 20, such that optimum grid excitation is maintained Within suitably narrow limits.

By way of specificv example and assuming the material to be treated is one whose dielectric constant and power factor increase during the heating cycle; when the-oscillator is first turned on, there will be a maximum flow of grid current in the solenoid which will stretch the coil 20. to reduce its inductance so reducing the grid current to the desired optimum value at which time the opposing force of the solenoid is balanced by the spring effect of the coil 20. As the heating cycle progresses, the grid current tends to. decrease, for reasons above discussed, with the result that the coil 20 tends to shorten, so increasing its inductance that the opposing forces exerted by the solenoid and the spring effect of the coil 20 areagainbalanced. The resulting increase in inductance of coil 2|] increases the grid drive to restore the, desired operating conditions. Therefore, during the heating cycle, the solenoid in effect continuously supervises. the. operatin conditions of the oscillator and effects adjustment ofv the inductance of coil 20. as and when necessary to maintain the prechosen. grid. excitation.

From an understanding of they invention as above described, it will now be apparent to those skilled in the art that othermodi ficationsof the invention. may be made within the scope, of, the appended claims, andthere have been. illustrated in. Figs. 2 and 3 two. such. modified form'sof the invention Referringfirst to Fig. 2, the, coil 20 has been illustrated as of. fixed shape. a'ndisize, and the solenoid 24. has been illustrated as. being effective to pull within the coil 20a. core 32 againstthabias. of aspring 33., The core 32' may preferably be in. the form of a cylinder of copper or other good nonemagnetic, conductor, inwhich case movement of the coreinwardly of the coil would decrease. the induc ance, thereof. The effect of movem'ent of the copper core 32' into the grid-coil is generally "the same as elongating the coil as described in connection with Fig. 1. It the core 32 be of magnetic material, such as powdered iron, the effect of moving such a core into the coil is to increase the inductance there-. of: use of a high-loss core is to be avoided. An arrangement of either type may be desired depending upon the particular oscillator and the initial position of the core with respect to the coil.

In accordance with Fig. 3, the inductance of the coil 28 is changed to maintain the grid excitation within a desired range by arranging the solenoid 24 to rotate a plurality of conducting plates 34, preferably of copper or other low-loss conductor, between the turns thereof. These conductors or plates upon moving into or out of interleaved positionwith respect to the turns of the coil 20 vary the inductance thereof automatically to compensate for changes in grid excitation due, for example, to variation in the electrical characteristics of a particular load during its heating cycle or due to the different kinds and sizes of load in difierent heating cycles.

What is claimed is:

1. In a dielectric heating system comprising an oscillator tube, a tuned anode circuit therefor including plates between which a dielectric load is disposed for heating, and a grid circuit with fixed. coupling to said anode circuit, the combination of reactance solely in said grid circuit and adjustable to vary the grid excitation of said tube, and a device energized in accordance with the direct-current component of said grid excitation to adjust said reactance to maintain the grid excitation substantially constant at a preset value despite changes of the electrical characteristics of said dielectric load.

2. In a dielectric heating system comprising an oscillator tube, a tuned anode circuit therefor including plates between which a dielectric load is disposed for heating, and a grid circuit with fixed coupling to said anode circuit, the combination of reactance solely in said grid circuit and whose magnitude determines the grid excitation of said tube, an electromagnetic 'device energized in accordance with the directcurrent component of said grid excitation, and means comprising the armature of said device and opposing spring means for adjusting the effective magnitude'of said reactance to'maintain the grid excitation substantially constant at a preset value despite changes of the electrical characteristics of said dielectric load.

3. In a dielectric heating system comprising an oscillator tube, a tuned anode circuit therefor including plates between which a dielectric load is disposed for heating, and a grid circuit with fixed coupling to said anode circuit, the combination of reactance solely in said grid circuit adjustable to vary the grid excitation of said tube and comprising a resilient deformable coil, an electromagnetic device energized in accordance with the direct-current component of said grid excitation, and means comprising the armature of said device and mechanical connection therefrom to said coil for adjusting the reactance of the coil in response to changes in magnitude of said direct-current component to maintain the grid excitation substantially constant at a preset value despite changes of the electrical characteristics of said dielectric load.

4. In a dielectric heating system comprising an oscillator tube, a tuned anode circuit therefor including plates between which a dielectric load is disposed for heating, and a grid circuit with nent of said grid excitation and having an armature connected to the free end of said coil to vary its length in sense to maintain the grid excitation substantially constant at a preset value despite changes of the electrical characteristics of said dielectric load.

5,. In a dielectricheating system comprising.

an oscillator tube, a tuned anode circuit therefor including plates between which a dielectric load is disposed for heating, and a grid circuit with fixed coupling to said anode circuit, the combination of reactance solely in said grid circuit adjustable to vary the grid excitation of said tube and comprising a coil of fixed shape and size provided with a movable inductance-varying element, spring means tending to move said element in one direction, and an electromagnetic device energized in accordance with the directcurrent component of said grid excitation and having an armature tending to move said element in reverse direction whereby the position of said element'is varied in sense to maintain the grid excitation substantially constant at a preset value despite changes of the electrical characteristics of said dielectric load.

8. In a dielectric heating system comprising an oscillator tube, a tuned anode circuit therefor including plates between which a dielectric load is disposed for heating, and a grid circuit with fixed coupling to said anode circuit, the combination of reactance solely in said grid circuit adjustable to vary the grid excitation of said tube and comprising a coil of fixed shape and size provided with an axially movable core, spring means tending to move said core in one direction, and an electromagnetic device energized in accordance with the direct-current component of said grid excitation and having an armature tending to move said element in reverse direction whereby the position of said element is varied in sense to maintain the grid excitation substantially constant at a preset value despite changes of the electrical characteristics of said dielectrio load.

7. In a dielectric heating system comprising an oscillator tube, a tuned anode circuit therefor including plates between which a dielectric load is disposed for heating, and a grid circuit with fixed coupling to said anode circuit, the combination of reactance solely in said grid circuit adjustable to vary the grid excitation of said tube and comprising a coil of fixed shape and size provided with vane structure movable between turns of the coil, spring means tending to move said vane structure in one direction, and an electromagnetic device energized in accordance With the direct-current component of said grid excitation and having an armature tending to move said element in reverse direction whereby the position of said element is varied in sense to maintain the grid excitation substantially constant at a preset value despite changes of the electrical characteristics of said dielectric load.

8. In a dielectric heating system comprising an oscillator tube, a tuned anode circuit therefor including plates between which a dielectric load is disposed for heating, and a grid circuit with fixed coupling to said anode circuit, the combination of a coi1 solely in said grid circuit and whose some siemfnduotanee is adj'ust'abie to vary the grief current or saidt'ube, and a: device fiavinga wine; ing energized: by" the direct-current component" or said grid" current and having" a membermov able in response to' said' energization to adjust the self-inductance of said coil" in same to maintain the grid excitation sirfistantial-ly constant at a; pr set": value despite changes of tire eieetrioai oli-a racteristics of said loadi Q: Anoscmatorsystem'oomprieingan'osciiiator tube; a grid" circuit" therefor; a tuned" anode oire cuit having fixed coupling to s'aidt'g'rid oircuitto provide. a feedoack' voltage; a rea'otan'oe solely in saictlgri'd' circuit acu'ustatiie" to vary ti'iegridfeircitation" or said' tube, and a device" energized: in accordance withthe direot currerit" component or saiti' grid excitation to vary sairr reactanee' in sense-tornaintain tnegrid exoitation substantial I'y constant at a: reset vaii'ie despite changes" tiieanod'e' oirouitloadupon said fee'dita'ek voltage.

110.. In at diei'ectric" fie'atin'g system comprising an osciiiator tube, a toned? anode circuit therefor having" fixed coupling" to the grid circuit thereof toprovi'dea feedback voltageand irrciuci inga capacitor having piat'es between which the dief'ectrictoad is disposed? for: heating; the cornbi'nation of a rea'ctancesoieiy' i'n triegridicirouit of saidtubeand adjustabie" to var'y'the magnitude or said feedback voit'age', and: a" protective device termite-tube" andthe ioadrenergfzed' bytiie direct current component of thegri'd excitation of said tube and having" a" responsive element mechanical-1y coupled to said rea'qtam'ce to" vary its mag"- nit'nde in' accordance with variations of said d'irent-current com onent and: insense; maintaim ih'g" said feedback voltage subetantiaiiy constant despite changes in We ereotrioal" el iaracteristioe ofsaici-dieiectricioad;

n. In a dielectric heating system: comprising. an oseiiiator tube" and a tuned anodecircuit therefor including pretes between which a dimetric load i's"'di"sposed roriieating", a feedback capacitance and acoil connected: in series with eachother from time: anode to the catiioo'e' otsaid tube-with an intermerfiate conneotionrto the grid or eaid tube; anti-a deviceenergizeciinaocordanoe with the oirectwnrrent component of the grid exeitationtowarytfie:s'eif indnctanoe'ors'aid'ooii te maintam the griefexcitation'substantfaiiycon stant for a fixed value of said reedbackfloapacf- 8*- tame and despite enarrgee oft'he ei'eetrieai" char-- actet'istics or" the dielectric ibad;

12. ina high fr'equency Heatingdeviceoftfie type comprising an oscillation generator supply said tube for automaticaiiy varying the axial iengti'r of said winding: continuously to maintain tiie grid excitation substantially constant for" a fixeo ooupiing" r said circuits and d'es it'e-a-fore"- eai'o changes of theloaci oharaeteristies:

fit a higii frquenoy'lieatirig device of the" typcdifipi'iirfg' an" oscifiati'on g'enerator sfipp1y= ing' a i'oae wi'iose electrical characteristics varyduring heating" including an osc'iiiator' tube hav: ing it; anodeand grid cireuitscoupled for generation'of oscillations, the oombi'nationor an in ducta-noe in grid circuit in the form or a Winding whose axial length is variable, means responsive tothe grid" current of said tube for antemat'ieaiiy varying the axial length of said winding continnousiy to maintain the grid ex citation substantially constant for a fixed coir- 151mg of said' circuits and de'spie aforesaid changes orthe-Ioaidcfiaiaeteristiosi References: Cited in the fiie of this patent UNITEfi STATES FA'I'ENTS 

